A new US study suggests that while the link between weight and causes of death varies considerably, being modestly overweight may actually lower death risk in a number of circumstances by providing the body with essential nutritional reserves during recovery from illness and major operations.

The study is published this week in the Journal of the American Medical Association (JAMA) and was carried out by Dr Katherine M Flegal, of the US Centers for Disease Control and Prevention (CDC), based in Hyattsville, Maryland.

Scientists from the CDC had already reported back in 2000 using data from national surveys, that risk of death from all causes was significantly lower in overweight people compared to normal weight, and significantly higher in the underweight and obese. The purpose of this new study was to get more insights into the findings, using specific mortality data with longer follow up for deaths among US adults during the year 2004.

Flegal and colleagues found that the link between weight and cause of death varied considerably. Being obese was linked significantly to increased rate of death from cardiovascular disease (CVD), being underweight was primarily linked to increased risk of death from non-cancer, non-CVD causes, and being overweight was linked to increased risk of death from diabetes and kidney disease together, but with reduced risk of death from other non-cancer, non-CVD causes.

Using records from the National Health and Nutrition Examination Survey (NHANES) and cause of death records for adults aged 25 and over during 2004, the researchers estimated the cause-specific excess deaths linked with underweight (BMI lower than 18.5), overweight (BMI 25 to under 30) and obesity (BMI 30 and over).

BMI is a person's weight in kilograms divided by their height in metres squared. For example, a person who stands 5 feet 9 inches tall (1.8 m) and weighs 150 pounds (68 kg) has a BMI of 22.5.

Based on total follow-up, the results showed that, compared with normal or healthy weight (BMI 18.5 to under 25):
  • Underweight was linked with increased mortality from non-cancer, non-CVD causes (23,455 excess deaths).

  • Underweight was NOT linked with cancer or CVD mortality.

  • Overweight was linked with significantly decreased mortality from non-cancer, non-CVD causes.

  • Overweight was NOT linked with cancer or CVD mortality.

  • Obesity was linked with significantly increased mortality from CVD (112,159 excess deaths).

  • Obesity was NOT linked with cancer, non-cancer, or non-CVD mortality.

  • Overweight and obesity combined were linked with increased mortality from diabetes and kidney disease (61,248 excess deaths).

  • Overweight and obesity combined were linked with decreased mortality from other non-cancer, non-CVD causes.

  • Obesity was linked to increased mortality from obesity-related cancers (13,839 excess deaths) but not other cancers.
The authors concluded that:

"The BMI-mortality association varies by cause of death. These results help to clarify the associations of BMI with all-cause mortality."

Reflecting on these results they wrote that:

"Some evidence suggests that modestly higher weights may improve survival in a number of circumstances, which may partly explain our findings regarding overweight."

"Overweight is not strongly associated with increased cancer or CVD risk, but may be associated with improved survival during recovery from adverse conditions, such as infections or medical procedures, and with improved prognosis for some diseases. Such findings may be due to greater nutritional reserves or higher lean body mass associated with overweight," they added.

The findings have received a mixed response among experts. Some say this means the optimal BMI range for normal or healthy needs to be revised upwards, while others say these figures do not say anything about quality of life.

Being overweight and obese increases risk of having many diseases that reduce quality of life. The public is not just interested in how long one is likely to live, but the extent to which quality of life is affected by weight.

Also, as the authors themselves mention in their reflections, some people have higher lean body mass that puts them in the higher BMI range, so perhaps the problem is with using BMI without qualifying the type of excess weight.

"Cause-Specific Excess Deaths Associated With Underweight, Overweight, and Obesity."
Katherine M. Flegal, Barry I. Graubard, David F. Williamson, Mitchell H. Gail.
JAMA 2007;298(17):2028-2037.
Vol. 298 No. 17, November 7, 2007

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Written by: Catharine Paddock